The preparation of oriented layers of crystallographically materials is critical in a number of technical areas. For example, high temperature superconductors often employ epitaxially oriented layers of, e.g., YBCO to achieve desired properties. Similarly, epitaxially oriented films such as conductive oxides, ferroelectric, ferromagnetic, piezoelectric, insulating, and semiconductive materials are desired in the areas of microelectric and opto-electric devices. To achieve the desired crystallographic orientation, e.g., epitaxial orientation, of materials such as YBCO, SrRuO3, PZT, Ba1−xSrxTiO3, La0.7Sr0.3MnO3, and Si, extensive work has focused on the underlying structure or substrate onto which the oriented layers are deposited.
For example, the production of coated conductors (superconductive tapes or films) has been referred to as a thick film process involving superconductive layers generally of at least one micron in thickness. In such a thick film process, it has been shown that YBCO thin films deposited directly on single crystal substrates (such as SrTiO3, LaAlO3, NdGaO3, yttria-stablized zirconia (YSZ), and MgO) can achieve critical current density (Jc) values of over 106 amperes per square centimeter (A/cm2) at 77 K. As large area single crystal substrates can be unavailable or prohibitively expensive, the use of polycrystalline substrates rather than single crystals has been desired. For amorphous or polycrystalline base substrates, the use of one or more suitable buffer layers to provide the necessary structural template for epitaxial growth of the superconducting layer was developed. For example, a YSZ buffer layer has been deposited by use of ion beam assisted deposition (IBAD) in which a YSZ layer is deposited in combination with irradiation from an ion beam directly on a deisred substrate during the deposition. Both Iijima et al., U.S. Pat. No. 5,650,378 and Russo et al., U.S. Pat. No. 5,432,151 have demonstrated deposition of in-plane textured YSZ buffer layers with IBAD, leading to YBCO thin films having excellent superconducting properties. Arendt et al., U.S. Pat. No. 5,872,080 described a coated conductor having the structure YBCO/Y2O3/YSZ/Al2O3/Ni alloy with a high critical current density (Jc) of about 1×106 A/cm2 and a high transport critical current (Ic) of from about 100 to about 200 A/cm. While the performance was satisfactory, the deposition of the YSZ layer was considered too slow for commercial production.
In U.S. Pat. No. 6,190,752 by Do et al., thin films of a material having a rock salt-like structure (e.g., MgO) were deposited by IBAD upon amorphous surfaces. In comparison to the deposition of YSZ, MgO with better in-plane texture can be rapidly deposited (about 100 times faster) through an IBAD process. The structures described by Do et al. included, e.g., YBCO/Y2O3/YSZ/MgO/MgO(IBAD)/Si3N4/Ni alloy with a NiO layer in between the YSZ layer and the MgO layer in most instances. Despite the improvement in processing speeds, the resultant Jc′s were only from about 0.025 to about 0.33 MA/cm2. A potential drawback in the use of such amorphous silicon nitride surfaces is that at the elevated processing temperatures needed to form the superconductive layer, silicon from the silicon nitride layer can migrate and react with other materials in the system. Additionally, Do et al. noted that the optimal thickness of their MgO layer likely corresponded to where the surface of the underlying substrate was fully covered by textured MgO crystallites and that with thicker depositions the texture begins to degrade. FIG. 1 shows a graph plotting the lattice tilt in degrees versus thickness of IBAD magnesium oxide upon an amorphous silicon nitride surface. It can be seen that the lattice tilt of IBAD magnesium oxide increases with increasing thickness.
Other groups have also taught the deposition of MgO upon amorphous substrates such as amorphous silicon nitride (Huhne et al., Materials Science Forum, vols. 408-412, pp. 1549-1554 (2002) and Groves et al., Physica C, vol. 382, pp. 43-47 (2002)), and oxidized silicon and Suprasil® fused silica (Huhne et al., Cryst. Res. Technol., vol. 35, pp. 419-425 (2000) and Huhne et al., Physica C, vol. 372-376, pp. 825-827 (2002)).
Further improvements in the structure and resultant properties of coated conductors have been sought. After extensive and careful investigation, applicants have found improvements in the preparation of composite substrate structures for subsequent growth of oriented layers thereon. The improvements have included the use of crystalline (e.g., nanocrystalline) materials upon which the MgO is deposited. Additionally, superconducting articles including such composite substrate structures have been developed.
It is an object of the present invention to provide composite substrate structures suitable for subsequent growth of oriented layers thereon.
Another object of the present invention is to provide composite substrate structures including flexible polycrystalline metallic substrates for subsequent deposition of, e.g. superconducting material such that resultant articles demonstrate desirable properties such as high Jc′s and Ic′s.
It is another object of the present invention to provide superconducting articles, especially superconducting articles including YBCO superconducting films.